6. Project Summary/Abstract The regenerative potential of skeletal muscle declines with age leading to an age-related decline in muscle bulk and strength and an increase in fibrosis. Our previous studies revealed that this declining regenerative potential in skeletal muscle is largely due to age-related changes in the functionality of adult muscle stem cells (satellite cells) and a tendency for those cells to adopt a fibrogenic fate rather than progress along the myogenic lineage. These changes were found to be due to age-related changes in a key signaling pathway, namely the Wnt signaling pathways, whose role we had characterized in the normal functioning of these adult muscle stem cells. Importantly, we have found that these age-related changes are reversible by exposing aged satellite cells to a youthful environment of by inhibiting Wnt signaling. The broad, long-term objectives of this research are to understand the molecular mechanisms of age- related decline in skeletal muscle regeneration and to develop treatments to enhance regeneration of aged muscle. The major focus of our current research in this area is to understand the reversible molecular changes that limit muscle stem cell function in the aged environment. As such, the Specific Aims of this proposal are: 1) To determine the transcriptional profile and specific epigenetic changes of satellite cells from adult and aged mice. For epigenetic studies, we will test for modifications of histones binding to the promoter region of the Delta gene, a gene that we had previously shown to be essential for satellite cell activation in adult animal but whose transcription is suppressed in aged animals. We will then test for both transcriptional and Delta epigenetic profiles in aged satellite cells exposed to experimental conditions that restore their youthful phenotype. 2) To determine the essential components of the Wnt signaling pathway that are responsible for the pleiotropic actions of Wnt on muscle stem and progenitor cells. We will specifically examine how BCL9, an essential component of Wnt signaling in Drosophila, functions in mammalian muscle stem cells to determine how Wnt signaling may lead to different cell fates under different conditions. 3) To explore the molecular interactions between the Wnt signaling pathway with two key longevity genes, SIRT1 and Klotho, and the functional significance of these interactions in terms of stem cell functionality in adult and aged organisms. Both SIRT1 and Klotho have been implicated as regulators of Wnt signaling. As such, we will use genetic mutants and pharmacological approaches to determine to what extent the Wnt-mediated effects of muscle stem cell aging are related to each of these pathways that play important roles in determining organismal aging. The results of these studies will lead to a better understanding of the biology of stem cell aging and the potential to enhance tissue regenerative potential in aged individuals.